1. Field of the Invention
The present invention relates to a light emitting device in which a light emitting element is mounted.
2. Background Information
An image reading device such as an image sensor is used in an apparatus such as a facsimile, a copier, and a hand scanner to read an original document. Generally used as such an image reading device is a contact image sensor which has a short optical path and is easily incorporated into an apparatus.
A contact image sensor includes a line lighting device for linearly illuminating an original document surface over a main scanning range. Conventionally known line lighting devices include a direct lighting type in which light emitting devices provided with a semiconductor light emitting element (hereinafter may be referred to as LED) are arranged in linear order to illuminate linearly, and an indirect lighting type in which a long light guiding member is used and the light which is introduced from a light emitting device arranged at the end surface of the light guide member is propagated along the length direction while repeating reflection in the inner surface and is emitted from the emission surface thereof.
When reading a black-and-white image, such a line lighting device typically uses single-color LEDs of two different emission colors in combination, such as a green LED and a red LED or a blue LED and a yellow LED. On the other hand, when reading a full-color image, single-color LEDs of three different emission colors; a red LED, a green LED, and a blue LED, are typically used as, for example, described in JP-2000-196820A.
Conventionally known materials for LEDs include; ZnSe and a nitride based semiconductor such as InXAlYGa1−X−YN (0≦X, 0≦Y, X+Y≦1) etc. for a green LED; GaAs, GaAlAs, AlInGaP etc. for a red LED; and ZnSe and nitride-system semiconductors such as InXAlYGa1−X−YN (0≦X, 0≦Y, X+Y≦1) etc. for a blue LED. In addition, a light emitting diode of InGaAlP-system as described, for example, in JP2002-237617A are known as a material for a yellow LED.
However, when reading a black-and-white image, if a green LED and a red LED are used, the output is needed to be increased due to their low sensitivity for reading. Moreover, a combination of a blue LED and a yellow LED is not practical because the output of a yellow LED having a composition as described above is too low compared with that of a blue LED.
In recent years it has been considered to use a LED of a fourth emission color in addition to red, green, and blue emission colors, to increase the color reproducibility when reading a full-color image. A yellow LED is however also impractical in such a case because of its low output.